2.x: make SingleMap not allow map function return null

[kojilin]

Found code like below can works

Single.just("1234").map(s -> null)
              .subscribe(s -> {
                  System.out.println(s);
              });

Because Single#just not allow null, so prevent mapper return null.

[codecov]

Codecov Report

Merging #5378 into 2.x will decrease coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5378      +/-   ##
============================================
- Coverage     96.15%   96.13%   -0.02%     
+ Complexity     5782     5780       -2     
============================================
  Files           630      630              
  Lines         41189    41189              
  Branches       5726     5726              
============================================
- Hits          39604    39597       -7     
+ Misses          621      619       -2     
- Partials        964      973       +9

Impacted Files	Coverage Δ	Complexity Δ
...reactivex/internal/operators/single/SingleMap.java	100% <100%> (ø)	2 <0> (ø)	⬇️
...ternal/operators/flowable/FlowableSubscribeOn.java	91.52% <0%> (-6.78%)	2% <0%> (ø)
...al/operators/observable/ObservableSampleTimed.java	91.66% <0%> (-5%)	3% <0%> (ø)
...ex/internal/operators/maybe/MaybeTimeoutMaybe.java	94.11% <0%> (-4.42%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableTakeLastTimed.java	95.37% <0%> (-3.71%)	2% <0%> (ø)
...reactivex/internal/operators/single/SingleAmb.java	96.36% <0%> (-3.64%)	9% <0%> (-1%)
...ternal/operators/observable/ObservablePublish.java	92.1% <0%> (-3.51%)	10% <0%> (ø)
...internal/operators/completable/CompletableAmb.java	94.91% <0%> (-3.39%)	10% <0%> (-1%)
...l/operators/observable/ObservableFlatMapMaybe.java	93.46% <0%> (-3.27%)	2% <0%> (ø)
...ternal/operators/completable/CompletableCache.java	96.96% <0%> (-3.04%)	23% <0%> (-1%)
... and 49 more

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[davidmoten]

LGTM, thanks!